How We Got On Land, Bone by Bone

How We Got On Land, Bone by Bone

Travel back far enough in your genealogy, and you will run into a fish.

Before about 370 million years ago, our ancestors were scaly creatures that lived in the sea, swimming with fins and using gills to get oxygen from the water. And then, over the course of millions of years, they began moving ashore, adapting to the terrestrial realm. They became tetrapods, a lineage that would eventually produce today’s amphibians, reptiles, birds, and mammals. As scientists have unearthed fossils from those early days, one lesson has come through ever more loud and clear: the transition was not a single leap. Instead, it was drawn out and piecemeal.

One of the most important of these fossils came to the world’s attention in 2006. Digging in the Arctic, a team of scientists found a 370-million-year-old creature they dubbed Tiktaalik. As I wrote at the time on the Loom,Tiktaalik belonged to a lineage of aquatic vertebrates called lobefins–a group that today includes lungfish and coelacanths. A number of anatomical features set lobefins apart from other fish, and show them to be more closely related to us and other tetrapods. They generally have stout fins that contain bones corresponding to the upper bones of our arms and legs. Some fossils of lobe fins don’t just have a bone corresponding to the humerus–the long bone attached to the shoulder–but the radius and ulna, too.

But even among lobefins,Tiktaalik was remarkably tetrapod-like. It had a distinct neck, for example, and its fins had additional limb-like bones. Along with bones corresponding to a humerus, radius, and ulna, it even had wrist-like bones that functioned as a joint, as they do in our hands. Without digits, Tiktaalik couldn’t grasp a branch with its fins. But it could do a decent push-up in the muddy shallows that it called home in the Devonian Period. (Neil Shubin, one of the discoverers of Tiktaalik, told the creature’s story in his 2009 book Your Inner Fish.)

The bones that Shubin and his colleagues described in 2006 came from the front half of Tiktaalik. Only now, eight years later, have Shubin and his colleagues unveiled the other half of this remarkable beast. And they’ve now stretched out the transition from fish to tetrapod even more.

In the comments on the OP link, there are questions about why Tiktaalik has eyes on top of its head. It really is no surprise that fish which evolved walking with fins on the sea-bed, would look upwards to watch out for predators.

A new species of shark that “walks” along the seabed using its fins as tiny legs has been discovered in eastern Indonesia, an environmental group said Friday.

The brown and white bamboo shark pushes itself along the ocean floor as it forages for small fish and crustaceans at night, said Conservation International, whose scientists were involved in its discovery.

Frogfish generally do not move very much, preferring to lie on the sea floor and wait for prey to approach. Once the prey is spotted, they can approach slowly using their pectoral and pelvic fins to walk along the floor.[6][8] They have two “gaits” that they can use. In the first they alternately move their pectoral fins forward, propelling themselves somewhat like a two-legged tetrapod, leaving the pelvic fins out. Alternately, they can move in something like a slow gallop, whereby they move their pectoral fins simultaneously forward and back, transferring their weight to the pelvic fins while moving the pectorals forward. With either gait, they can only cover short stretches.

Fish have the genetic machinery necessary to make fingers, but it is not switched on, a study suggests.

The research in Plos Biology journal sheds light on how fish evolved into the earliest land animals millions of years ago.

For fish to make the transition to land, an existing DNA architecture had to be “hijacked” in order to make digits, the researchers said.

In order to do this, they took genes from fish and inserted them into mice.

It was already known that the genes for limbs are found in fish but how they evolved to form digits remained unclear.

To unravel the genetics, the authors used the zebrafish as a model. But other scientists said that zebrafish were not a useful species for studying limb evolution.

Lead author Joost Woltering from the University of Geneva, Switzerland, said that he was interested in the “longstanding evolutionary question – how did limbs actually develop out of ancestral fish fins?”

In order to answer this, Dr Woltering and his colleagues looked at the genetics of fin and limb developments in zebra fish and mice.

He was particularly interested in the division of the hand and arm (or digits), which does not exist in fish fins and “is considered one of the major morphological innovations during the fin-to-limb transition”.
‘Architect’ genes

Tetrapods, the first four-legged creatures to walk the Earth, evolved from water to land over 380 million years ago in an era known as the Devonian period, often referred to as “the age of fish”.

Fish and land animals both have clusters of genes called HoxA and HoxD and both are known to be essential in fin and limb development.

These Hox genes are sometimes referred to as “architect genes” as they are involved in making many of the physical structures animals possess.

However, when these Hox genes from fish were placed into mouse embryos, the genes that result in the arm were switched on but not the genes responsible for the hand or the digits.

This suggests that the genetic information needed to make tetrapod limbs was already present in fish before the tetrapods evolved.

“During embryogenesis it is key that developmental genes are switched on at exactly the right time and right place to ensure the development of a complete, coherent good functioning adult organism,” Dr Woltering told BBC News.

“The most surprising result is we found [DNA in fish] which is almost identical to the higher order DNA structure that we found in the mouse.”

Another important conclusion of the study is that fish fins are not equivalent to the tetrapod hand and digits. Instead, the evolution of digits in land animals involved the repurposing of existing genetic infrastructure.

One of the co-authors of the study, Prof Denis Duboule, also from the University of Geneva, said: “Altogether, this suggests that our digits evolved during the fin-to-limb transition by modernisation of an already existing regulatory mechanism.”

Other researchers in the field say that the study contains some flaws.

Jennifer Clack, from the Cambridge University Museum of Zoology, who was not involved with the study, said using the zebrafish as a model for the experiments was a bad choice.

“We know that this animal, and by inference its relatives… lack some of the developmental stages that make digits in tetrapods,” she explained.

Prof Clack added that other finned fish such as Polydon [paddlefish] “do have that mechanism, operating in a similar way to that in tetrapods, to make a complex fin skeleton”.

This suggests, she said, that the zebrafish at some point lost the ability to make digits.